1. Field of the Invention
The present invention relates generally to noncorrosive steam pressure safety release valves, and more particularly, to a valve disc for use in power generating nuclear fission reactor plants and associated equipment.
2. Description of the Prior Art
Steam pressure relief valves are critical devices in any type of high pressure, hot water, or steam producing units. In a nuclear power plant there are standards and demands that must be applied to the design and fabrication of such valves. The function of the steam pressure relief valve is shown conceptually in the illustration of FIG. 1. The principles of nuclear power generation are well established and understood. The nuclear reactor 10 is located in a containment building 11 and contains uranium or plutonium fuel elements in a core arrangement. Through neutron absorption and nuclear fission (chain reaction) of the uranium or plutonium, large amounts of energy are released in the form of heat which is utilized to generate electricity.
In a boiling water reactor, the chain reaction boils the moderator-water in the core. Steam is thus produced inside the reactor 10. Steam pipes 12 carry the steam from the reactor 10 to the turbines 13. In producing electricity, a nuclear plant""s steam turbines and electric generators work like those in a fossil fuel plant. The steam produced by a reactor spins the blades of the plant""s turbines 13, which drive the generators. Many plants have combination turbines and generators called turbogenerators.
After steam has passed through a plant""s turbines 13, it is piped to a condenser 14. The condenser 14 changes the steam back into water 15. Water 15 is then returned to reactor 10 by pump 16. There are several conditions that may occur in the nuclear plant which require a relief of steam pressure from the system. For this reason, a steam relief safety valve 17 is used in the steam pipe system. The purpose of the valve 17 generally is to relieve steam pressure in excess of 1150 pounds per square inch (xe2x80x9cpsixe2x80x9d) to prevent overpressurization of the nuclear reactor and subsequent damage and potential shut down. This overpressure could be caused by several events such as a main electrical breaker closure which eliminates the resistance to the power turbines, or any misadventure that might cause overpressure with subsequent excessive revolutions per minute (xe2x80x9crpmxe2x80x9d) of the turbines. To prevent excessive rpm to the turbines, the steam power to them is shut down with the pressure then traveling back to the reactor. The valve 17 is designed to then exhaust the excess steam above 1150 psi.
The present invention relates to two-stage safety relief valves as part of the nuclear pressure relief system in boiling water reactors to provide automatic depressurization, if needed, for breaks in the nuclear system so that the low pressure coolant injection and the core spray systems can operate to protect the fuel barrier. This two-stage valve is referred to as a main steam relief valve which is installed in the main steam lines. The main steam relief valves are distributed among the four main steam lines so that an accident cannot completely disable a safety, relief, or automatic depressurization function. Of the nuclear plants that use this valve, each reactor (typically 2-3 per plant) has approximately thirteen of these valves along the main steam line.
The typical main steam relief valve contains a disc which engages a seat when the valve is in the closed position. The disc and seat are presently made of stellite, a cobalt-based metal containing chromium and carbon. The main problem for the nuclear energy plant is risk of overpressure to the nuclear reactor because of the occasional failure of the main steam relief valve to relieve at the designated pressure. This condition is due to corrosion bonding of the stellite disc to the stellite seat as a result of exposure to radiation, heat, moisture, and oxygen from the steam. Plating the disc with platinum has improved the function of the disc but has not cured the problem of corrosion and bonding of the valve to the seat.
Valve discs made of ceramics might be resistant to this corrosive and bonding effect in nuclear plants. However, oxide-based ceramics would not be useful because the presence of oxide causes the ceramic to easily corrode in the presence of oxygen, high temperatures, alkalis, and acids. On the other hand, silicon carbide ceramic is known to be highly resistant to corrosion under extreme conditions. However, it is a complicated process to sinter silicon carbide because it has to be sintered at high temperatures in an inert atmosphere. The stellite valve disc in present use in nuclear reactors has a complex shape which would make construction of a similar valve out of silicon carbide highly impractical. The stellite disc has a conical shape on one end and a cylindrical shape on the opposite end. The cylindrical portion has three different external diameters and two different internal diameters which would make the construction of this valve from silicon carbide unfeasible. To my knowledge, no alternative valve disc composed of silicon carbide has ever been made available for a nuclear power plant heretofore.
The present invention has solved these and other shortcomings by providing a silicon carbide valve disc for use in a main steam relief valve. The cylindrical portion of this disc has a uniform external diameter and a uniform internal diameter and is feasible to construct entirely from silicon carbide or similar carbide ceramics.
One aspect of the present invention relates to a main steam relief valve for use in a boiling water reactor nuclear power plant. In particular, the valve of the present invention is a valve for regulating steam pressure in a nuclear power plant, comprising: a hollow disc including a cylindrical first end having a uniform external and internal circumference, a conical opposite end, an external surface, and an internal surface, said conical portion adapted to receive a push-rod on the internal surface and a valve seat on the external surface, wherein said disc comprises a ceramic carbide. The valve disc has a cylindrical first end of uniform external and internal circumference, a conical opposite end, an external surface, and an internal surface. The cylindrical first end has a plurality of groves on the external surface to prevent rotation of the disc and to allow steam to exhaust. The conical portion is highly polished on the external surface to prevent the formation of cracks. The conical portion is adapted to receive a push-rod or a steel plate and push-rod on the internal surface, and a valve seat on the: external surface. The valve disc is constructed entirely of 90% or greater oxide-free silicon carbide, boron carbide, titanium diboride, or boron nitride. The disc is highly resistant to destruction by physical impact by virtue of the thickness of its walls and its conical portion. In a preferred embodiment, the disc is constructed from superfine powders of silicon carbide, hot pressed to rough shape, premachined, sintered at high temperature, diamond wheel ground, and polished.
An advantage of the present invention is to provide a valve disc for a nuclear power plant that is resistant to corrosion during operation of the nuclear power plant.
Another advantage of the present invention is to provide a valve disc for a nuclear power plant which will not bond to its valve seat during operation of the nuclear power plant.
Another advantage of the present invention is to provide a valve disc for a nuclear power plant that is resistant to destruction by physical impact.
Yet another advantage of the present invention is to provide a valve disc for a nuclear power plant that is constructed of silicon carbide, boron carbide, titanium diboride, or boron nitride with purity of 90% or greater.